Section 8.3.2 of the HDC1000 datasheet says that the battery status is reported in the TRES field (Bit 11). I think that it should say BTST field as shown in 8.6.3.
Also, I don't get BTST to change signs until the battery voltage is 2.5V (not 2.8V). And, it will only read that signal that way once. It requires a reset to read it again. (I am checking the configuration register directly after a read of temperature and humidity).